Separator membranes used in alkaline electrochemical cells for the purpose of physically separating the anode and cathode to prevent dendrite growth between the two electrodes while at the same time permitting ionic flow therethrough are well known in the art. Separator membranes possess characteristics of low electrolytic resistance and high resistance to oxidation and corrosive attach in alkaline solutions at high temperatures. Furthermore, the separator membrane must have sufficient mechanical strength to prevent dendrite growth or treeing between the electrodes and to withstand the rigors of battery fabrication. Thin membranes of thermoplastics such as polyethylene and polypropylene are commonly used as separators since they are very stable in an alkaline electrolyte environment of alkaline electrochemical cells. Other thermoplastic films may be used. As is known in the art, such materials cannot function as separators in their basic thin film form since they are ionically impermeable. Accordingly, they are treated mechanically to provide pores for ionic permeability or they are chemically or radiation grafted so that they can function as separators by allowing hydroxyl ion flow therethrough. Both crosslinked and non-crosslinked thermoplastic separator membranes are known in the art.
Separators frequently have contiguously associated with one side thereof a thin mat or felt of absorbent material for the absorption and retention of electrolyte. Typically, the absorbent is associated with the side of the separator such that the absorbent mat is positioned between the separator and the anode of the electrochemical cell. In some instances, the mat or felt associated with the separator may be positioned between the separator and the cathode; may be associated with both sides of the separator membrane; or a separator membrane may be associated with both sides of a layer of absorbent felt in accordance with the design requirement of a particular electrochemical cell. The absorbent material also possesses the characteristics of resistance to attach by corrosive materials such as hydroxides; resistance to oxidation; low ionic impedance; and resiliency under compression.
In electrochemical cells having cylindrical shaped anodes, the separator membrane or the separator membrane/absorbent material composite will have a tubular shape for receiving the cylindrical anode. Heretofore, tubular shaped separator membranes or separator membrane/absorbent composites have been manufactured by heat sealing, impulse heat sealing, or adhesive joining. A sheet of separator membrane or separator membrane/absorbent mat composite material would be sized and rolled into tubular form. The two ends along the length of the rolled tube would be overlapped or brought together in the form of a tab. The lap or tab would then be heat sealed, impulse heat sealed or glued. The seals thus formed result in a waste of material and a loss of effective ionically permeable area of the tubular separator.
Heat sealing has a particular disadvantage in that the heat required to form the heat seal tends to overheat the area adjacent the heat seal. That is, there is a transition area adjacent the heat seal where the melting process starts to take place. This results in a high electrolytic resistance in the area of the tube adjacent the heat seal and thus results in further reduction of the effective ionically permeable area of the tubular separator.
Impulse heat sealing does not cause as great an area adjacent the seal to be overheated as conventional heat sealing. Nevertheless, an area adjacent the seal is still overheated thus reducing this areas electrolytic resistance resulting in the reduction of the effective ionically permeable area of the tubular separator.
A further disadvantage of heat sealing and impulse heat sealing arises in the manufacture of ionically permeable tubular separators formed from a composite membrane/absorbent material. Where there is a great difference in the thickness of the two materials forming the composite, it is difficult to get a good effective seal.
Gluin or adhesive joining is generally an inefficient and difficult manufacturing process to perform.